The production of diisocyanates by reaction of diamines in the gas phase is described, for example, in EP 0289840 B1. The diisocyanates that are formed in a tubular reactor are thermally unstable at the reaction temperatures of up to 500° C. A rapid cooling of the reaction gases after the phosgenation reaction to temperatures below 150° C. is therefore necessary in order to prevent the formation of unwanted byproducts due to the thermal decomposition of diisocyanate or due to a further reaction. In EP 0289840 B1 or EP 0749 958 B1, for this purpose, the gaseous mixture continuously leaving the reaction space and which contains, inter alia, diisocyanate, phosgene and hydrogen chloride, is introduced into an inert solvent, e.g. dichlorobenzene. It is disadvantageous in this method that the flow rate with which the gas mixture is passed through the solvent bath must be selected to be relatively low, since at excessive velocities, solvent and the compounds dissolved therein would be entrained. In a subsequent step, the liquid compounds would have to be separated off from the gas. A further disadvantage is that, owing to the low flow rate and a low heat transfer term, large solvent vessels must be used in order to achieve the cooling.
In addition, methods are known which, for cooling the reaction gases, use heat exchangers and/or expand the gases in a vacuum (DE 10158160 A1). The disadvantage of heat exchangers is that, because of the poor heat transfer, large exchange surfaces, and therefore large heat exchangers, are required for effective cooling. In addition, deposits of solids on the relatively cold surfaces of the heat exchangers can occur because of side reactions of the gas mixture such as, e.g., decomposition, polymerization or precipitation.
EP 1 761 483 B1 attempts to decrease the residence time between end of reaction and cooling zone by a region having a reduced flow cross section being situated between the reaction zone and the zone in which the reaction termination is effected.
The reaction termination zone (termed “quench”) described in the application WO2007/014936 A2 is a region in which the hot product gases are cooled rapidly by spraying in a quenching liquid. Possible quenching liquids mentioned are solvents, isocyanate mixtures and solvent/isocyanate mixtures. Spraying in a quenching liquid for cooling the reaction mixture and selective dissolution of the formed diisocyanate in the solvent, wherein a first separation into a liquid phase and a gas phase having predominantly phosgene and hydrogen chloride as constituents proceeds is mentioned. The two phases are thereafter fed to a corresponding workup. Possibilities for optimization of this method step are not considered.
A further development method for rapid cooling of the gaseous reaction mixture is offered by spraying one or more quenching liquids into the gas mixture continuously flowing from the reaction zone into the downstream quenching zone, as mentioned in WO 2011/003532 A1 and described in more detail in EP 1403 248 B1.
Spraying in quenching liquid using at least two spray nozzles arranged at the intake of the quenching zone is disclosed in EP 1 403 248 B1. In this case, as quenching liquids, organic solvents are suitable, or a mixture of various organic solvents which do not react with the diisocyanate formed, as described in EP 1 403 248 B1. A solution of the diisocyanate formed in a suitable organic solvent can also be employed, which reduces the amount of solvent used. The diameter of the quenching zone can be greater or smaller than the diameter of the reaction zone. The quenching of the reaction gases can proceed in a single stage and in multiple stages. In this document, reference is made to the fact that the method of spraying in the quenching liquid into the hot reaction gas proceeds in such a manner that contact of the reaction gas with the cold wall is avoided. As a result, the formation of deposits is avoided.
This system is optimized in EP 1 935 875 B1 in that, for termination of the reaction, the reaction mixture is conducted out of the reaction space through a cooling section into which liquids are sprayed in such a manner that the direct cooling in the cooling section proceeds in a single stage manner (that is to say giving only one condensation mixture) into two or more series-connected cooling zones. The diisocyanate generated is obtained in this case in a common condensation mixture. This mixture is preferably collected in a liquid collection vessel, arranged beneath the cooling section. This condensation mixture can be discharged for separating off the isocyanate produced or, preferably after cooling has been performed, in part recirculated to one or more cooling zones of the cooling section. A disadvantage of this use of the “crude product mixture” from the gas phase phosgenation can be soiling of described cooler occurring before entry into the quench. Causes thereof can be unwanted byproducts or polymer compounds from the phosgenation reaction. Also reference is made in EP 1935875 B1 to the fact that the nozzle design of the cooling liquid must be selected in such a manner that the hot reaction gas entering into the quench does not contact the relatively cold walls of the cooling zones and to avoid deposits of solids. In addition to the condensation mixture in the collection vessel, downstream of the cooling section, the gas containing at least hydrogen chloride, phosgene, possibly solvent, and the isocyanate produced, is obtained. This gas stream is taken off from the collection vessel and fed to a scrubbing column and is there substantially freed from the isocyanate fractions thereof. Preferably, this scrubbing proceeds in counterflow to the solvent. The wash phase thus obtained, consisting of diisocyanate and for the most part of solvent, is used in a preferred embodiment as quenching liquid of the first cooling zone of the cooling section. The residual gas from the scrubbing column consists substantially of phosgene, hydrogen chloride and solvent. These vapors leave the column overhead, wherein by means of partial condensation, in a preferred embodiment, via two condensers having different coolant temperatures, the solvent fraction is substantially retained and is recirculated to the column as partial condensate. The residual gas obtained thereafter, which substantially consists of phosgene, hydrogen chloride and solvent residues, is subsequently further treated in a manner known per se, as described, e.g., in EP 1 849 767 B1.
In EP 1 935 876 A1, the use of different suitable quenching liquid streams is likewise mentioned. In this case, reference is made to the use as quenching liquid of the scrubbing liquid from the gas scrubbing of the vapors leaving the condensation vessel downstream of the quench. The reaction is carried out here adiabatically. In the examples according to the invention, the reactors are described as correspondingly insulated, in order to avoid as substantially as possible heat losses. In the non-insulated reactor of the example not according to the invention, in the experiment deposits form on the walls, which lead to the termination of the experiment owing to pressure increase.
According to the teaching of EP-A-1 362 847, temperature fluctuations and asymmetries in the temperature distribution lead to the formation of byproducts which lead to deposits and blockages in the gas phase reactor.
EP 2 196 455 A1 also refers to a plurality of cooling zones in the quenching stage. Here, reference is made for the first time to the integrated combination of the cooling zones of a plurality of reactors with a quenching stage. EP 2 196 455 A1 also refers to the fact that neither mixing spaces nor reaction space permit cooling surfaces, which can give rise to condensation with the consequence of deposits.
WO 2010/063665 A1 refers to a possible problem of the quenching variants cited hitherto. If at least a part of quench liquid is taken off from the collecting vessel after the quench, that is to say the diisocyanate crude product solvent mixture, there is the possibility that solids can be present which can block the quench nozzles. Various techniques are described such as, e.g., centrifugation, distilling off the liquid fraction provided for the quench, or filtration. In order to set the temperature of the selected quench stream for the object in question, the stream can be cooled or heated by means of a heat exchanger.
In WO 2010/115908 A2, a certain embodiment of the quench is disclosed. In order to prevent secondary reactions of the reaction gas during or after the quenching stage, the quench nozzles and the arrangement thereof are designed in such a manner that a substantially complete wetting of the wall in the quench region proceeds. The entire reaction mixture is included thereby. Quenching liquids proposed are solvents and also mixtures with isocyanate or crude mixture of the phosgenation reaction, optionally after particle removal. This document discloses that, owing to inadequate wall wetting in the quenching region, condensate can form on cooler wall zones, which, on account of excessively slow drainage, can lead to secondary reactions with solid deposits.
Said descriptions do not consider a fundamental problem in operating the reactor (including the cooling zone (“quench”)) in phosgenation in the gas phase. It is observed during operation, with time, solid deposits on the reactor walls where they are no longer wetted with liquid. The origin of the solid is not completely known. Without wishing to be bound by theory, the supposition is expressed that these are higher molecular weight byproducts from the reaction. These deposits occur more intensively in the region of the quench nozzles. There, the deposits can grow so greatly that a marked constriction of the effective free cross section of the reactor occurs and a correspondingly high pressure drop. Finally, it can even occur that the production needs to be ended, namely when the pressure drop is too high, or there is no longer free passage. FIG. 1 illustrates this situation: a product gas stream 3 passes through a reactor 100 and is cooled by a quench liquid 4. The dashed line at the height of the quench nozzle illustrates the boundary between reaction zone and quench zone: the reaction zone is above the quench nozzle and the quench zone is below the quench nozzle. Above the dashed line, along the periphery of the reactor (that is to say before entry into the actual quench zone), formation of deposits 1000 occurs.